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Boletín de la Sociedad Chilena de Química

versión impresa ISSN 0366-1644

Bol. Soc. Chil. Quím. v.46 n.2 Concepción jun. 2001 


S.A. MOYA 4*.

1Escuela de Química, Facultad de Ciencias, Universidad Central de Venezuela,
Caracas 1020-A, Venezuela.
2Centro de Investigación y Desarrollo de Radiofármacos, Facultad de Farmacia,
Universidad Central de Venezuela, Caracas, Venezuela.
3Centro de Investigaciones Químicas, Facultad de Ingeniería,
Universidad de Carabobo, Valencia, Venezuela.
4Departamento de Química Aplicada, Facultad de Química y Biología,
Universidad de Santiago de Chile, Santiago, Chile.

(Received: July 24, 2000 - Accepted: November 22, 2000)

* To whom correspondence should be addressed.


Reported here are the influence of the reaction conditions variation (carbon monoxide pressure, P(CO), rhodium content, [Rh], and temperature) on the catalytic reduction of nitrobenzene to aniline by [Rh(COD)(2-pic)2](PF6) (COD = 1,5-cyclooctadiene, 2-pic = 2-picoline) immobilized on poly(4-vinylpyridine) in contact with 80 % aqueous 2-ethoxyethanol under water-gas shift reactions conditions (RDGA, CO + H2O ¤ CO2 + H2). The turnover frequencies for the production of aniline displayed a first order dependence on the pressure of CO in the 0 - 1.9 atm range, a nonlinear dependence on rhodium content on the 0.9 - 12.2 wt. % range indicating that the active species have different nuclearities. The temperature dependence followed a segmented Arrhenius plot in the 70 ­ 130 °C ranges. These data are discussed in terms of a possible mechanism.

KEYWORDS: Nitrobenzene reduction, WGSR, rhodium catalyst.


Se reporta en el presente estudio la influencia de las variaciones de las condiciones de reacción (presión de monóxido de carbono, P(CO), concentración de rodio, [Rh], y temperatura) en la reducción catalítica de nitrobenceno a anilina por el complejo [Rh(COD)(2-pic)2](PF6) (COD = 1,5-ciclooctadieno, 2-pic = 2-picolina) inmovilizado en poli(4-vinilpiridina) en contacto con 80 % de 2-etoxietanol acuoso bajo condiciones similares a la reacción de desplazamiento del gas de agua (RDGA, CO + H2O ¤ CO2 + H2). Las frecuencias de conversión en la producción de anilina muestran una dependencia de primer orden con respecto a la variación de la presión de CO en el intervalo de 0 - 1,9 atm, una dependencia no lineal con respecto al contenido de rodio en el intervalo de 0,9 - 12,2 % (p/p) indicando que las especies activas tienen diferente nuclearidad. La dependencia de la temperatura presenta una curva de Arrhenius segmentada en el intervalo de 70 ­ 130 °C. Estos datos son discutidos en términos de un posible mecanismo.

PALABRAS CLAVES: Reducción de nitrobenceno, RDGA, catalizadores de rodio.


A convenient method to obtain aminoarenes compounds, which are of the great interest in the industrial manufacture and laboratory 1), is the catalytic carbonylative reduction of nitroarenes (Eq. 1) by transition metal complexes under condition similarly to the WGSR 2-13).

C6H5-NO2 + 3 CO + H2O Æ C6H5-NH2 + 3 CO2 (1)

On the other hand, functionalized cross-linked polymers have been largely used to immobilize or heterogenised transition metal catalysts. The main goal of this approach is the preparation of catalytic systems displaying the good activity, selectivity and reproducibility shown by homogeneous catalyst, combined with the easy separability and recovery characteristic shown by heterogeneous catalyst 14).

Catalytic reduction of nitroarenes under condition similarly to the WGSR by transition metal complexes heterogenised on nitrogen-functionalized polymers has been reported 15-17). However, a detailed kinetics study has not been carried out, so far.

The objective of the present study was to examine the quantitative details related to the catalytic reduction of nitrobenzene to aniline by [Rh(COD)(2-pic)2](PF6) complex immobilized on poly(4-vinylpyridine) under condition similarly to the WGSR, as a function of variable parameters, namely P(CO), [Rh] and T.


Materials. The 2-picoline (Aldrich) was distilled from KOH. [Rh(COD)Cl]2 was obtained from Aldrich. Nitrobenzene was distilled in H2SO4 (1 M) and redistilled in CaCl2 prior to use. Water was doubly distilled. 2-Ethoxyethanol (Aldrich) was distilled from anhydrous stannous chloride. Poly(4-vinylpyridine) (P(4-VP)), 2% cross-linked was used as provided by Reilly Industries. All gases and gas mixtures N2, He/H2 (91.4% / 8.6%, v/v), CO/CH4 (95.8% / 4.2%, v/v) and CO/CH4 / CO2/H2 (84.8% / 5.1% / 5.3% / 4.8%, v/v) were purchased from BOC Gases and were used as received. The rhodium [Rh(COD)(2-pic)2](PF6) complex was prepared as reported by Denise and Pannetier 18).

Instrumentation. Analyses of Rh concentration in catalysis solution were performed on a Perkin-Elmer Lambda 10 spectrophotometer and on a GBC Avanta atomic absorption (AA) spectrophotometer operated in the flame mode. Gas samples analyses from catalysis run were performed as described in detail previously 19, 20) on a Hewlett-Packard 5890 Series II programmable (ChemStation) gas chromatograph fitted with a thermal conductivity detector. The column used was Carbosieve-B (80-100) mesh obtained from Hewlett-Packard using He/H2 mixture as the carrier gas. Analyses of liquid phase reaction product were done on a Hewlett-Packard 5890 Series II programmable gas chromatograph fitted with 3% OV-101 Supelcoport (mesh 80-100) column and flame ionization detector using orto-xilene as an internal standard, and identified by co-injection on a Hewlett-Packard 5890 Series II programmable gas chromatograph-mass spectrometer fitted with Pona capillary column (50 m).

Catalyst preparation. A 0.5 g sample of P(4-VP) and a known amount (typically, 1.0 x 10-4 mol) of the rhodium complex were stirred for 120 h in 10 mL of 2-ethoxyethanol/H2O (8/2, v/v) until almost all the rhodium was extracted by the polymer from the solution as marked by the presence of a colorless clear solution above the polymer beads. The yellow polymer was filtered, washed with 2-ethoxyethanol/H2O (5 mL) to remove the unabsorbed rhodium which concentration was determined by UV Visible and AA spectroscopies. This procedure allowed knowing the amount of rhodium supported by subtraction of the amount of unabsorbed rhodium from the initial rhodium concentration.

Batch reactor procedure. Kinetics runs were conducted in all-glass reactor vessels consisting of a 100 mL round bottom flask connected to an "O" ring sealed joint to a two-way Rotoflow Teflon stopcock attached to the vacuum line 10). In a typical run, the loaded polymer beads and 10 mL of aqueous 2-ethoxyethanol were added to the glass reactor vessel, then the solution was degassed by three freeze-pump-cycles. The reaction vessel was charged with CO/CH4 mixture at the desired CO partial pressure (typically 0.9 atm), then suspended in a circulating thermostated glycerol oil bath (typically 100 °C) by a given time (typically 3 h) with analog controller (Cole-Palmer, Model 71). The specified temperatures were maintained at ± 0.5 °C by continuously stirring the oil bath as well as the reaction mixture which was provided with a Teflon-coated magnetic stirring bar. After reaching a constant WGSR activity 19) a given amount of (typically 0.26 mL (2.4 x 10-3 mol)) of nitrobenzene was added to the reaction vessel. Subsequent to addition of nitrobenzene, the glass reactor vessel was charged with CO/CH4 at 0.9 atm partial CO pressure and placed in the heated oil bath for 3 h at 100 C for the nitrobenzene catalytic reduction runs and at a given CO pressure, temperature and time for the kinetics runs of the same catalytic reaction. At the end of a given reaction time gas samples (1.0 mL) were removed from the reactor vessel in a manner similar to described in detail for the WGSR catalytic test 19) and analyzed by GC. Also, liquid samples were removed and analyzed by GC and GC-MS. The CH4 was used as internal standard to allow calculation of absolute quantities of CO consumed and CO2 produced, during a time interval. In addition, calibration curves were prepared periodically for CO, CH4, H2, and CO2, and analyzing known mixtures checked their validities.


Research in our group has shown the catalytic activity toward both the WGSR19) and nitrobenzene reduction (NBR) 21) by rhodium(I) amino complexes of the type [Rh(COD)(amine)2](PF6) (2 wt %) immobilized on P(4-VP) (0.5 g) in contact with 10 mL of 80% aqueous 2-ethoxyethanol under 0.9 atm of CO at 100 °C. For example, the WGSR catalytic activity defined as TF(H2) (mol gas (mol Rh x 24 h)-1) production followed the order: 4-picoline (11.9) > 3-picoline (9.9) > 2-picoline (5.7) > pyridine (5.4) > 3,5-lutidine (5.2) > 2,6-lutidine (3.3) under the above described reaction conditions.

On the other hand, the same used rhodium(I) complexes immobilized on P(4-VP) in contact with 80% aqueous 2-ethoxyethanol and nitrobenzene also generates a system which shows a catalytic activity on the selective carbonylative reduction of nitrobenzene to aniline under CO (0.9 atm)/H2O condition at 100 °C. The GC-MS results showed that aniline was obtained with selectivity over 99% at 11 - 36% nitrobenzene conversion range in 3 h. The aniline production (mmole/3 h) depend on the nature of the amine and decrease in the following order: 2-picoline (0.65) > 4-picoline (0.59) > 3-picoline (0.56) > pyridine (0.49) > 3,5-lutidine (0.38) > 2,6-lutidine (0.34). The WGSR and NBR catalytic activity observed in these systems were explained in terms of a fine balance between electronic and steric effects introduced by the methyl groups.

The positive effect of amine basicity should be noted, nitrobenzene conversion increases with the increase of pKa of the amine ligand with the only exception of the 2,6-lutidine which presents strong steric constraints that lower the catalytic activity. These results suggest a critical steric parameter, which can be viewed as the effect of competition for binding to the catalytic center, which is affected more by steric constraints than by electronic effects. For example, the coordination (cycloaddition) of the nitro group with the catalyst (a crucial first step) 22, 23) can be affected by the presence of the two methyl groups and consequently decreasing the catalytic activity. Surprisingly, the role of coordinate amine in these catalysts is different from that in the WGSR19) by the same Rh/amine/P(4-VP) system. For example the sterically hindered 2-picoline, which shows a low catalytic activity for the WGSR, is the most effective ligand for reduction of nitrobenzene among the amines ligand tested 21).

Further, in the case of the catalytic NBR described above, the formation of aniline, the only organic product (detected by analyzing the catalysis liquid phase) and CO2 the only gas product (detected by analyzing the catalysis gas phase) matched stoichiometrically as required by (Eq. 1). So, due to the simplicity and speed of the analysis of gaseous samples, results reported here are based on milimole (mmol) aniline formed on basis of CO2 production and these were periodically confirmed by analyzing the liquid phase. We had previously reported in detail the consistency of this analytical method 10). For this Rh(2-pic)/P(4-VP) system the effects of varying the reaction conditions were explored. These results are summarized in Table I - III and they represented the average value of duplicate runs deriving for the same experimental conditions. The calculated activity defined as TF(aniline) was reproducible to within less than 10% for a series of experimental runs. In addition, the TF(aniline) were determined for short periods (3 h or less) where P(CO) and [nitrobenzene] were essentially constant, diminishing by less than 15% overall. Hence, possible shifts in P(CO)- and [nitrobenzene]- dependent equilibria among the catalyst component during a run were minimized owing to the near constancy of P(CO) and [nitrobenzene].

Effect of carbon monoxide pressure. The effect of varying the CO pressure is summarized in Table I. Figure 1 displays a plot of the TF(aniline) vs. P(CO) at different temperatures, where TF(aniline) were calculated from the expression: mol of aniline / (mol of Rh x 24 h)-1. In addition, the log-log plots of these curves have slope values of 1.1 ± 0.1; 1.1± 0.1; and 1.0 ± 0.1 at 80, 100 and 120 °C, respectively. This clearly establishes that the reaction is first order in [CO]. Similar kinetics behavior was reported by us in the homogeneous catalysis of nitrobenzene to aniline by cis-[Rh(CO)2(2-pic)2]PF 6 under WGSR conditions 24).

Based on the first order in [CO] we suggests a possible mechanism in which the rate-limiting step (k2) formation of the rhodium nitrobenzene [P(4-VP)nRh(2-pic)2(CO)(C 6H5NO2)]+ complex is preceded by coordination of CO, (step (1c) in Scheme I) e.g.

[P(4-VP)nRh(2-pic)2 ]+ + CO ® [P(4-VP)nRh(2-pic)2(CO)] + (2)

[P(4-VP)nRh(2-pic)2 (CO)]+ + C6H5NO2 ® [P(4-VP)nRh(2-pic)2]+ + products (3)

The rate law for such behavior would be:

Rate = k1k2P(CO)[C 6H5NO2][Rh]tot (4)

where [Rh]tot = [P(4-VP)nRh(2-pic)2]+ + [P(4-VP)nRh(2-pic)2(CO)] + and k1 includes the solubility of CO in the medium. The above expression (Eq. 4) can be reduced to:

TF(aniline) = k1k2P(CO)[C 6H5NO2] (5)

For this kinetics model, plots of TF(aniline) vs. P(CO) should be linear with slopes of k1k2[C6 H5NO2] and zero intercept. Indeed these plots are linear at 80, 100 and 120 °C with nearly zero intercept value as predicted by (Eq. 5).

Fig.1. Plot of log TF(aniline) vs. log P(CO)
at 80, 100 and 120 °C, 0.5 g P(4- VP),
[Rh] = 2.0 wt. %, 0.26 mL of nitrobenzene,
[nitrobenzene]/[Rh] = 25, in 10 mL of
2-ethoxyethanol/water, 8/2, v/v.

It has been postulated the cycloaddition of nitrobenzene (C6H5NO2) and nitrosobenzene (C6H5NO) to a metal carbonyl double bond followed by formation of CO2 and the corresponding metallacycle 25). The latter acts as catalytic intermediates during the catalytic reduction of nitrobenzene.

Effect of rhodium concentration. The effect on the catalytic activity of altering the Rh content on the polymer was also investigated (Table II). Runs were carried out for a series of different rhodium concentrations over the range 0.9 to 12.2 wt. % keeping [nitrobenzene]/[Rh] ratio constant at 25 through the variation of the amount of nitrobenzene in the 0.13 - 1.57 mL range. A typical run involved determining TF(aniline) as a function of [Rh] at P(CO) = 0.9 atm at 100 C. Figure 2 shows the TF(aniline) values vs. [Rh] plot. An increase in [Rh] from 0.9 to 8.1 wt. % resulted in a decrease in TF(aniline), followed by nearly constant values at higher [Rh]. The results indicate that catalyst activity is not first order in the [Rh] range of 0.9 to 12.2 wt. % and suggest that the active species may be present in several forms having different nuclearities (mononuclear and polynuclear). This suggestion is strongly supported by the characterization FT-IR and XPS results reported for this [Rh(COD)(2-pic)2] (PF6)/P(4-VP) system 21).

Fig. 2. Plot of TF(aniline) vs. [Rh] for P(CO) = 0.9 atm at 100 C, 0.5 g of P(4-VP), 0.13 - 1.47 mL of nitrobenzene, [nitrobenzene]/[Rh] = 25, in 10 mL of 2-ethoxyethanol/water, 8/2, v/v.

Effect of temperature. To determine the activation parameters, TF(aniline) values for this Rh/P(4-VP) system were measured at various temperatures (Table III). Figure 3 displays the TF(aniline) values vs. T plot for [Rh] = 2.0 wt. % at P(CO) = 0.9 atm. Arrhenius type plot of TF(aniline) values were nonlinear in the 70 to 130 °C range, giving concave curves suggesting a lower "activation energy" (Ea) at temperatures below 100 °C (Ea1 = 4.7 Kcal.mol-1) than temperatures > 100 °C (Ea2 = 11.7 Kcal.mol-1). Arrhenius plots, which are concave upwards, suggest a change in the rate-limiting step between two competitive reactions 26).

The effect of the variation of the temperature can provide information also about the possible implication of mass transport phenomena in the course of the hydrogenation reaction 27). As seen in Figure 3 there is a clear increase in the apparent activation energy in the high temperature range. If diffusion (mass transport phenomena) is taking place during this reaction, one should observe a contrary effect. This is to say, a decrease of the apparent activation energy to a one-half value of that obtained at low temperature. In our case the apparent activation energy at high temperature is four times as high as that for the low one.

Fig. 3. An Arrhenius plot for the data obtained for Rh/P(4-VP) catalyzed nitrobenzene reduction in 10 Ml of 2-ethoxyethanol/water, 8/2, v/v, 0.26 mL of nitrobenzene, [nitrobenzene]/[Rh] = 25 under P(CO) = 0.9 atm.

Mechanistic considerations. The evaluation of the mechanism for aniline formation by the catalytic system prepared from [Rh(COD)(2-pic)2](PF6) immobilized on P(4-P) in 80% aqueous 2-ethoxyethanol and nitrobenzene in CO shows a few key features. First, aniline was the only organic product detected. Second, the carbon monoxide pressure study indicates that the reaction is first order in [CO] and is independent in [RNO2]. Third, the rhodium concentration and temperatures studies suggest a mono-polynuclear equilibrium between active rhodium species, under nitrobenzene reduction conditions. Fourth, the early reported FT-IR, UV/Vis reflectance, EPR, SEM, and XPS studies suggested the presence of mononuclear cationic and polynuclear anionic carbonyl compounds anchored to the nitrogen-functionalized polymer as reaction intermediates 21). Fifth, catalytic schemes for the reduction of nitrobenzene to aniline have been proposed in which nitrobenzene cycloaddition to a metal carbonyl complex is an important first step 23). Given the above, the reaction mechanism depicted in Scheme I is proposed for the reduction of nitrobenzene to aniline catalyzed by mononuclear cationic Rh species:

Scheme I

In Scheme I, coordination of CO to [P(4-VP)nRh(2-pic)2]+ complex would give [P(4-VP)nRh(2-pic)2 (CO)]+ (1c). The cycloaddition (1d) of the nitro group (-NO2) of the nitrobenzene (RNO2, R = C6H5) compound to the Rh-CO bond followed by elimination of CO2 leads to the formation of rhodium nitrosobenzene [P(4-VP)nRh(2-pic)2(Q2 -ONR)]+ complex through the deoxygenation of rhodium nitrobenzene [P(4-VP)nRh(2-pic)2(Q 2-O(CO)RNO)]+ species (1e). The formation of [P(4-VP)nRh(2-pic)2(Q 2-O(CO)RNO)]+ specie (1d) as the rate-limiting step would rationalize first-order dependence of TF(aniline) on P(CO). Insertion of one CO molecule to the Rh-O bond of the nitrosobenzene complex (1f) forms [P(4-VP)n Rh (2-pic)2(Q 2-O(CO)NR)] + complex.

Decarboxylation of the rhodium [P(4-VP)nRh(2-pic)2(Q 2-O(CO)NR)]+ (1g) generates a rhodium-nitrene complex [P(4-VP)nRh(2-pic)2(NR)] + and CO2. Hydrogenation (1h) of this rhodium-nitrene specie probably by the [P(4-VP)nRh-H] species formed under conditions similarly to the WGSR 20, 28) affords aniline, CO2 and [P(4-VP)nRh(2-pic)2]+ complex to get catalytic cycle closed. Analogous cycle may be proposed for the polynuclear Rh immobilized species.


The following conclusions have been draw from the influence of the reactions conditions studies on the nitrobenzene reduction by [Rh(COD)(2-pic)2](PF6) complex immobilized on aminated polymer P(4-VP) in contact with aqueous 2-ethoxyethanol under conditions analogous to those for the WGSR: the Rh(2-pic)/P(4-VP) system shows a first order on the rate on P(CO) and non linear dependence on the rate on Rh content. The effect of variation of Rh content and temperature on catalytic activity can be explained on the base of the presence of at least two kind of immobilized rhodium species with different nuclearities and oxidation states.


The present work was carried out under the research programs CONICIT-Venezuela (Proy. S1-95001662), DICYT-USACH and FONDECYT-CHILE (SAM) and CODECIHT-UC, (PB). We thank Reilly Industries INC by donating the poly(4-vinylpyridine) cross-linked polymer (lot N. 70515AA).


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